1. Field of the Invention
This invention is a bellows for conversion between thermodynamic energy and mechanical work. In an engine, this bellows can be used in place of a piston and a cylinder. One such application is the internal-focusing heat engine operating in a Stirling thermodynamic cycle so as to convert solar energy into electric power.
2. Description of the Prior Art
The most common type of metallic bellows now used is a circumferentially-pleated thin-walled expansible tube which is basically a single piece of metal. Fabricating the pleats of such a bellows usually introduces so much strain and deformation that the material must be selected more for its ductility than for its strength. Therefore, a product results which lacks stiffness and is quite limited with respect to allowable operating pressures, expansion range, temperature and reliable life.
The most efficient type of heat engine known is the type in which temperature, pressure and other thermodynamic variables theoretically follow what is called a Carnot cycle. A practical engine capable of coming close to ideal thermodynamic efficiency is the Stirling engine such as the internal focusing engine invented by Daniels and Finkelstein (U.S. Pat. No. 3,117,414). However, the achievement of high thermodynamic efficiency tends to increase the non-thermodynamic losses, particularly the friction and leakage losses.
Heretofore, some bellowslike apparatus have been used in engines to reduce friction or leakage losses. Examples are bellowslike seals by Gross and Frazier and the "roll sock seal" by Meijer. However, these examples reduce friction in only a limited and indirect way because they do not eliminate the sliding friction between a piston and a cylinder. Replacing a piston with a one-piece circumferentially-pleated bellows would introduce intolerable restrictions on pressure, volume, etc.
In its simplest form, a Stirling engine has two pistons, one to control the temperature of the working fluid which is generally a gas (usually hydrogen, air or helium) and one to transfer mechanical power (usually to extract it). Some form of coupling with piston movement is required so that cyclic thermodynamic changes (temperature, pressure, volume, etc.) in the gas can occur with desired phase relationships as in the case where the engine is a prime mover. If this coupling takes the form of a mechanical cranking mechanism such as referenced above (U.S. Pat. No. 3,117,414), the engine is called a kinematic Stirling engine. If it takes the form of tuned dynamic (elastic and inertial) forces as in the case of an engine invented by Beale, it is called a free piston Stirling engine.